(1) Field of the Invention
This invention concerns films and prepregs including UV curable resins that are useful in the manufacture of flexible PCBs as well as to methods for using the films and prepregs in the manufacture of printed circuit boards.
(2) Description of the Art
Currently, standard flex-rigid multilayer manufacturing processes use flexible polyimide sheets to overlay the copper structures and noflow prepregs. Such standard flex-rigid laminates use expensive materials. Moreover, the manufacturing method is very complex and requires multiple manual processing steps.
A prior art method for constructing build-ups for an inner-layer flexible multilayer is shown in FIG. 1. The build-ups are formed stepwise by:                structuring a polyamide foil layer including a polyamide planar core (10) a copper layers (11) on each planar polyamide sheet surface and coverlayers (18) overlying the copper layers (11);        milling a c-staged prepreg layer (12) to form a space (13);        applying a cover layer (14);        laminating the partial layup;        depth milling at the core top and bottoms (shown by black triangles (16));        stacking-up and lamination of the multilayer; and        removing the rigid portion of the cores overlying space (13) by milling.        
The prior art laminate includes a plated through hole (PTH) (30). The area of the plated through hole in the vicinity of the structured polyamide foil layer creates reliability problems because it requires plasma cleaning. In addition, issues with inadequate metallization of the PTH are common and undesirable barrel cracks can form due to the high thermal expansion of the polyimide and the cover layer in comparison the PTH thermal expansion. Moreover, during the lamination process, little to no pressure is applied to cover layer (14) and polyimide foil layer (10). As a result, PTFE inlays are placed in the stack by hand to increase the stack laminating pressure.
Instead of using the polyamide foil layer (10), a cheaper FR4 material layer can be used instead such as is shown in FIG. 2. However the disadvantage of using the FR4 material is that the resulting layup can only be bent a few times before it becomes damaged. There is a need, therefore, for improved methods for manufacturing flexible multilayer inner-layer laminates that are used to manufacture printed circuit boards that do not suffer from one or more of the problems mentioned above.